The invention relates to an apparatus for the metallurgical aftertreatment of premelted metals having a cylindrical ladle receiving the melt and closed by means of a cover and one or more electrodes passing through the cover and intended for heating the melt via electric arcs for producing steels with carbon contents below 0.02% or steels with narrow carbon tolerances, the steel being premelted in a smelting unit and subsequently being alloyed, rabbled and homogenized in an aftertreatment unit, the melt being heated via electric arcs between the melt and electrodes.
Apparatuses of this type have been part of the state of the art for a long time (thus, for example, GB periodical Steel Times, February 1978, pages 205-211). Their development arose from the desire, in the smelting of metals, to transfer substantially all the metallurgical work from the actual smelting unit to a subsequent treatment process. Only melting down would still be carried out in the smelting unit. Particularly in electric steel plants for the smelting of steel, this makes it possible to achieve considerable reductions in furnace time and consequently increased output. This results in so-called ladle metallurgy, referred to as secondary steelmaking in English-Language Literature, in which the metal from the premelting unit is run off into a ladle and subjected to aftertreatment in this. The main functions of after-treatment in steel production are:
steel alloying, PA1 establishing exact analyses, and PA1 analytical and temperature uniformity in the ladle.
To compensate for the temperature losses in the ladle, which are unavoidable during treatment and caused particularly by the addition of solid alloying metals and the scavenging of the steel melt by means of inert gases introduced via floor-level blowing bricks, the aftertreatment ladle is providing with a ladle-heating device, usually in the form of an electric-arc heating device, usually in the form of an electric-arc heating system. When heating of this type is used, a cover similar to that of an arc furnace is located above the ladle. By means of one or more electrodes (usually operated with alternating current) suspended on supporting arms and guided through a cover in electrodes guides, the steel melt can be heated via the electric arcs which are generated. Electric-arc heating can take place both under atmospheric pressure, in which case it is not necessary to seal the ladle off from the cover, and under a vacuum, in which case the ladle is closed so as to be vacuum-tight relative to the atmosphere and the electrodes are guided through the cover via vacuum gaskets. Round graphite electrodes which can be subjected to high current intensities and which generate an easily adjustable arc are used as electrodes according to the state of the art. They have a diameter of 300 to 500 mm, depending on the ladle size and the desired heating capacity. However, the electrodes are used up during operation as a result of oxidation with the atmosphere. Thus, as regards a ladle-heating system equipped with three 450-mm electrodes operated with alternating current, the electrode consumption is approximately 0.5 kg per ton of treated steel. At a price of approximately 6.--CM per kg of electrode, this corresponds to costs of approximately 150,000.--DM in the case of a monthly aftertreatment of approximately 50,000 tons of steel. Apart from these costs, it is not possible to use graphite electrodes in the aftertreatment of steels having very low carbon contents (0.02% C) or steels with a narrow range of analyses relating to the carbon content. The inert scavenging gas introduced into the melt through floor-level blowing bricks to homogenize the steel melt, especially after the addition of an alloying agent, makes the melt bubble in such a way that the electrodes normally "burning" just above the melt surface come in contact with the melt, and so much carbon enters the melt from the graphite material of the electrodes that the prescribed low or narrowly defined carbon contents are exceeded and the steel can therefore no longer be used for the intended purpose.